Meteoroids, which are solid pieces of extraterrestrial debris, can stray from their orbits in outer space and be captured by Earth's gravity. Most come from the asteroid belt lying between the orbits of Mars and Jupiter, about 400 million kilometres from the Sun. Bodies within this belt range from dust particles up to small planets hundreds of kilometres in diameter (asteroids). Rarely, meteoroids may come from the Moon, Mars or comets.

What is the main difference between meteors and meteorites?

meteors: the initial solid particle (meteoroid) can be too small to survive its flight through Earth's atmosphere and burns up completely when heated by friction with air, to give a momentary streak of light

meteorites: larger meteoroids survive their fiery ordeal and land on the Earth's surface. Very small meteoroids may remain intact or melt to form glassy droplets which rain down on Earth's surface as micrometeorites. Tiny dust particles get rid of heat as quickly as it is applied, so do not burn up and fall gently on the Earth's surface.

Meteors

The small particles which become meteors typically range from the size of a grain of sand up to the size of a pea. When heated to incandescence by friction with air in our atmosphere, they burn up completely at heights of about 80 km - 130 km. The trail of glowing, electrically-charged gases that surrounds and streams away from the meteoroid is called a meteor. These brief, bright streaks of light are sometimes called 'shooting stars'. Meteors tend to arrive in 'showers' of many individual meteors at definite times throughout the year. They are given the name of the constellation which forms their backdrop (e.g. the Leonids from Leo, November 14 - 17). These constellations are much further away than the showers themselves.

Meteorites

Meteoroids that survive their journey through our atmosphere land on the Earth's surface as meteorites. They sometimes break up into several pieces while still in flight or when they hit the Earth. Large meteoroids can form brightly glowing fireballs (bolides) seen clearly in daylight, accompanied by spectacular light and sound effects. They can travel at very high speeds, typically from about 15 km - 70 km per second, the fastest ones (over 30 km per second) are destroyed on impact with the atmosphere. Most are heated for less than 10 seconds while they fall. Meteorites range in size from pea-sized pieces up to large masses many tonnes in weight.

Meteorites which are found after their fireball was witnessed are called 'falls', but if found accidentally long after their arrival they are called 'finds'. A famous Australian meteorite fall occurred on 28 September 1969 at Murchison, near Shepparton, Victoria. Meteorites are named after the locality where they have fallen (town, county, property, river, valley, mountain etc.).

About 500 tonnes of meteoritic debris falls to Earth every day, much of it as fine cosmic dust and micrometeorites that fall into the sea. About 500 meteorites of reasonable size would hit the Earth's surface every year, of which 150 would fall on dry land, and less than 10 would actually be found. Many fall in rugged, inaccessible landscapes and have a poor chance of being located. The largest known meteorite weighs 60 tonnes and still sits where it fell, at Hoba, Namibia.

Meteorites are extremely important to science, as they help us to better understand the origins and composition of the Solar System. They are our major source of extraterrestrial material apart from moon rocks retrieved by space missions.

Meteorite craters

Large meteorites have enough energy to make an impact crater when they hit the Earth, and pieces of meteorite may be scattered around and within the crater. There are about 350 proven and possible meteorite impact structures scattered throughout the world. The Canon Diablo ('Devil's Canyon') crater in Arizona, USA, has a 1.5 km diameter and is 170 m deep. The meteorite which made this crater may have been 30,000 to 100,000 tonnes, and an estimated 10 m - 25 m in diameter. Sometimes a meteorite is a large one, travelling very fast, and with a large amount of energy. Such meteorites may explode and vaporise before hitting the ground, and the shock wave will excavate a crater and partially melt surrounding rocks or give them shock deformation structures.

Types of meteorites

Depending on how much metal or stony silicate material is present, meteorites can be irons, stony-irons, or stones. These may represent the centre (irons), inner (stony-irons) and outer parts (stone) of small planetary bodies which collided and broke up in the asteroid belt, or material that failed to clump together to make planetary bodies when the Solar System first formed.

The minerals which make up the bulk of meteorite composition include seven common Earth minerals and three found only in meteorites:

Earth minerals

olivine - magnesium iron silicate

pyroxene (hypersthene, bronzite) - iron magnesium silicate

plagioclase feldspar - calcium sodium aluminium silicate

magnetite - iron oxide

hematite - iron oxide

troilite - iron sulphide

serpentine - magnesium iron silicate with water

Minerals found only in meteorites

taenite - nickel-iron (high nickel)

kamacite - nickel-iron (low nickel)

schreibersite - iron nickel phosphide

Small amounts of other Earth minerals can be present in meteorites, but over 20 minor minerals occur only in meteorites and not on Earth. Altogether, about 295 different minerals have been found in meteorites.

Stone meteorites

Stones are the most common type of meteorite and can be of moderately large mass such as the 0.5 tonne stone from Wildara, Western Australia. They consist mainly of the silicate minerals olivine and pyroxene with feldspar and scattered nickel-iron.

Stones are further subdivided according to whether small, spherical, concretionary mineral structures (chondrules) are present (chondrites) or absent (achondrites). Most stones are chondrites, of which there is an unusual type, the carbonaceous chondrites (e.g. The 'Murchison', Victoria) which have enormous scientific and popular interest because of their strange bituminous smell, high organic content and amino acid-like chemicals. Rare classes of achondrite stones (e.g. the Shergottites) are basalt lavas thought to have come from Mars.

Iron meteorites

Irons are the second most common type of meteorite and have the largest masses, such as the 12 tonne 'Mundrabilla', Western Australia iron. Irons are made of dense silvery nickel-iron metal alloys (taenite and kamacite) and have a range of nickel contents from about 4% (Hexahedrite Class) to 6 - 12% (Octahedrite Class) to over 20% (Ataxite Class). When cut, polished and etched with an acid/alcohol solution, some (the Octahedrite Class) show a characteristic criss-cross pattern of intersecting platy nickel-iron crystals known as Widmanstatten structure.

Stony-iron meteorites

Stony-irons are the least common type of meteorite. One type (Pallasite Class) has about equal amounts of the silicate mineral olivine, and nickel-iron metal (green glassy olivine crystals set in a continuous meshwork of silvery nickel-iron), while another type (Mesosiderite Class) is a broken up (brecciated) mixture of pyroxene with nickel-iron. A well-known New South Wales example of a Pallasite is the 'Molong' stony-iron.

Recognising meteorites

Meteorites have a brown, black or grey outer fusion crust of magnetite and hematite, but inside they look quite different. The interiors of irons show bright silvery metal, and stones show speckled cream, brown or grey silicate grains with scattered metal specks and veins.

Stony meteorites usually have 'boxy' shapes with small, rounded hollows like thumb prints on their outer surfaces and rounded corners and edges.

Irons and stony-irons can have regular, smoothed shapes, but can also be twisted 'shrapnel' shapes if they have been broken up and deformed.

Irons are very heavy - a 10 cm cube of an iron would weigh about 7.5 kg to 7.8 kg, and a similar cube of a stony-iron would weigh between 4 kg - 6 kg. Stones (chondrites) are brittle and less heavy - a 10 cm cube would weigh between 3 kg and 4 kg, while carbonaceous chondrites (black and crumbly, often smelling like bitumen or camphor), would weigh between 2.2 kg and 3.5 kg. Achondrite stones are brittle, may have a shiny black fusion crust, and have a pale grey or cream interior with little or no metal. A 10 cm cube of an achondrite would weigh about 3.0 kg - 3.5 kg. Irons and stony-irons are strongly attracted to a magnet, while chondrite stones are attracted weakly and achondrites are almost non-magnetic.

Some large meteorites

Meteorites vary from small pebbles to single objects or groups of many tonnes. The world's largest single iron meteorites are the 60 tonne 'Hoba' from Namibia, Africa, and a 36 tonne mass from the Cape York, Greenland. The largest falls of stones are the 'Allende', Mexico carbonaceous chondrite (over 2 tonnes of pieces) and the 'Norton County', USA achondrite (over 1 tonne of pieces). The world's largest single stony-iron is the 'Huckitta' Northern Territory pallasite (1.4 tonnes).

Meteorites worldwide

There were 24,858 named meteorite falls and finds up to June 2002. These are listed in the worldwide meteorite catalogue at the Natural History Museum, London and localities include:

Antarctica: 19,884

Algeria: 452

Australia: 578

Africa (north-west): 407

Libya: 1302

Oman: 511

United States of America: 1346

False meteorites

Other objects can often be mistaken for meteorites - ironstones and other heavy rocks and minerals; melted materials (slag, coke) from metal smelting and manufacture of glass, bricks and coke; ball bearings and manufactured metal alloys. Always take any suspected meteorite to your nearest museum for identification and comparison with known meteorites. New meteorites, previously unknown to science, have been discovered in this way.

Some well-known smaller craters (diameter)

Martian meteorites

A rare group of stony meteorites (less than 20 are known) may have come from the planet Mars. Most of these have been found in Antarctica.

They were originally named 'SNC's', after the names of the three meteorite subgroups into which they were first classified: Shergotty (S), Nakhla (N) and Chassigny (C). However, a new subgroup of Martian meteorites was recognised when a meteorite from the Allan Hills in Antarctica (ALH 84001) was also found to be from Mars.

All of the Martian meteorites are igneous rocks, having solidified at or below Mars surface. This makes them difficult to distinguish from many similar rocks on Earth. However there are several pieces of evidence that point to an origin on Mars.

Evidence for a Martian origin

It is believed that all of the Martian meteorites landed on Earth as fragments from large meteorites or asteroids that crashed into Mars between 180 and 1300 million years ago.

The meteorites are most likely to have come from the planet Mars because:

The meteorites are relatively young in age (< 1300 million years) compared to the Solar System and other meteorites (~ 4500 million years).

The mineral grains within them have a different oxygen isotopic composition compared to the Earth and Moon.
Trapped pockets of gases within at least one of the Martian meteorites have the same chemical and isotopic composition as those of the Martian atmosphere sampled by the Viking landers of 1976.

All Martian meteorites are formed from igneous rocks, so they must have formed on an earth-like planet or one of its moons. Venus and Mars are the only other likely candidates. Since the atmosphere on the planet Venus is very dense, it causes any ejected rock produced by impact to melt and vaporise due to frictional heating. This means that the planet Mars is left as the most likely origin of ejected meteorite fragments.

The Dar al Gani 476 meteorite

A Martian meteorite called 'Dar al Gani 476' was found in the Libyan Desert on May 1 st, 1998. It is an achondrite (shergottite) stony meteorite, mainly composed of the minerals olivine and plagioclase, and is believed to have originated as a basalt on the planet Mars. The total known weight of this meteorite is 2.015 kg, but the specimen shown here weighs only 0.358 grams and has dimensions of 1.5 cm x 1.5 cm x 0.2 cm.